Electric discharge device



July 11, 1939.

a. A. MORTON ET AL.

ELECTRIC DISCHARGE DEVICE 2 Sheets-Sheet 1 Original Filed Feb. 2:8, 1935INVENTOES.

ear eiMori/om LasfLieEF (01y,

15y omgmefllg .WTTOENEK July 11, 1939. a. A. MORTON ET AL 06 ELECTRICDISCHARGE DEVICE Original Filed Feb. 28, 1935 2 Sheets-Sheet 2I/vvE/vmw: 6190119 flMvntvm LeaLwEFLary,

WOW

Patented July 11, 1939 UNITED STATES ELECTRIC DIS George A. Morton, Ha

CHARGE DEVICE ddon Heights, and Leslie E. Flory, Oaklyn, N. J assignorsto Radio Corporation of America ware , a corporation of Dela-Application February 28, 1935, Serial No. 8,630 Renewed May 12, 1937 5Claims.

Our invention relates to electric discharge devices and moreparticularly to devices of the type wherein amplification of a primaryelectron stream, such, for example, as is emitted from a thermioniccathode or from a photo-sensitive surface exposed to light, isaccomplished through utilization of the phenomenon of secondaryemission.

If an electrode is subjected to electron bombardment, it will emitsecondary electrons. The ratio of the number of secondary electrons tothe number of primary electrons depends, in part, upon the character ofthe bombarded surface and upon the potential diiference between thesurface and the source of the electrons. This ratio can be madeconsiderably greater than unity. For example, a ratio of three or moresecondary electrons to one impinging electron is readily obtainable withmetallic surfaces treated in known ways and subjected to discharges atpotentials. of 300 to 400 volts. Since the emitted electrons exceed theimpinging electrons in number, the electrodes emitting them,hereinafter, will usually be referred to as "multiplying electrodes.

If the secondary electron current, in turn, is caused to impinge withsufficient velocity upon a further electrode with a suitably treatedsurface, the ratio of secondary emission from the second multiplyingelectrode may also be greater than unity. Hence, one is able to obtainwith n multiplying electrodes in cascade, for example, an amplificationof the original or primary electron current equivalent to theamplification per electrode raised to the nth power. A million-foldamplification may be obtained in a single device.

Prior art devices, wherein the phenomenon of secondary emission isutilized for amplification, have proved to be unreliable andinefficient, principally because of incomplete and uncertain control andutilization of the secondary electron streams. For example, in theoperation of a device constructed according to French Patent 582,428(Dapsence et al.), the same electrostatic fields are utilized forelectron acceleration and for determining the paths described by theprimary and secondary electrons. It has been found that, in general, itis very difficult to control the paths of the electrons in a tube ofthat type in such manner as to cause all of the electrons from any onesource to fall upon the desired target.

Another disadvantage of previous electron multipliers, of typesexemplified by the French patent, resides in the fact that the field inthe neighborhood of each multiplying electrode, which serves to draw offthe secondary electrons, must necessarily be weak, Because of the smallmagnitude of the field, it is impossible to draw large currents fromthose electrodes. An attempt to overcome this defect by increasing thepotential of the succeeding electrode, with the object of increasing theaccelerating field in the neighborhood of the preceding multiplyingelectrode, generally results in a decreased gain since some of theprimary electrons which would normally strike the preceding multiplyingelectrode are drawn past it to one of the following electrodes. As aconsequence, because of the space-charge limitation of current, it isimpossible to maintain linearity between the input excitation or thecurrent from the primary source, and the output current, except forextremely weak output currents.

It is, accordingly, an object of our invention to provide an electricdischarge device, utilizing secondary electron emission, wherein maximumgain per stage is obtained and linearity may be obtained between inputexcitation and output current.

Another object of our invention is to provide an amplifier or electronmultiplier of the secondary electron emission type in which thesecondary electron stream from each emitter is con- 0 centrated and isdirected accurately to the desired target, and interference between thevarious secondary electron streams is minimized.

Another object of our invention is to provide an amplifier or electronmultiplier wherein there is practically no loss of secondary electronsand in which the best conditions for amplification, or other desiredresults, can easily be obtained by external potential adjustments.

Another object of our invention is to provide an amplifier or electronmultiplier that is efiicient and reliable in operation and in which theamplification obtainable is very great as compared with theamplification obtainable with a thermionic amplifier of usual type.

Another object of our invention is to provide a device of the typedescribed that may be used for substantially any purpose for whichthermionic tubes of present types are used, such,

for example, as an amplifier, a demodulator, an a oscillator, a combinedoscillator and demodulator, etc.

Another object of our invention is to provide a combined phototube andamplifier that shall be responsive to the very highest frequenciesencountered in television transmitting apparatus.

A still further and more specific object of our invention is to providea device of the type described that lends itself readily to massproduction methods.

The foregoing objects and other objects ancillary thereto we prefer toaccomplish by the provision of separate electrostatic fields for theproduction of secondary electrons from one electrode and the focusing ofthe electrons upon the next succeeding electrode. In particular, in apreferred embodiment of our invention we cause electrons emitted from aprimary source, either photo-electric or thermionic in character, toimpinge upon a surface capable of secondary emission at a velocitysufficiently high to produce secondary emission having a ratio greaterthan unity to the primary electron stream. Through utilization of anelectron lens system, the secondary electron stream thus produced isdirected against another similar surface at a still higher positivepotential whereat further secondary electrons are produced. In each caseelectron lenses are made use of for the purpose of concentrating anddirecting the primary electrons toward the first secondary emitter, thesecondary electrons from the first emitter toward the second emitter andso on. This process may be repeated a number of times within the samecontainer and the final greatly amplified stream of secondary electronsis collected by an output electrode.

More specifically stated, a preferred embodiment of our invention isconstituted by an evacuated zig-zag-shape container wherein aresupported a plurality of successively disposed electrodes and the innerwalls of which, between the electrodes, are provided with conductivecoatings which, when supplied with proper potentials, function aselectron lenses.

As will be pointed out hereinafter, the zig-zag shape of the containeris not absolutely essential but it readily lends itself to manufacturingmethods and it has been found most convenient for experimental purposes.

Further, in accordance with our invention, each of the successiveelectrodes and each of the coatings, going to make up the electronlenses, is provided with a lead extending exteriorly of the container,by means of which proper potentials may be applied thereto in operation.

Because of the fact that we utilize electron lenses for the purpose ofconcentrating and directing electrons from place to place in ourimproved device, we are enabled to employ it in apparatus wherein thepresence of electromagnetic focusing fields would be detrimental.

The novel features which we believe to be characteristic of ourinvention are set forth with particularity in the appended claims. Ourinvention itself, however, both as to its organization and method ofoperation will best be understood by reference to the followingdescription taken in connection with the accompanying drawings, inwhich:

Figure l is a view in perspective of a preferred embodiment of ourinvention, portions of the wall being broken away to more clearlydisclose the electrode structure and disposition,

Fig. 2 is a diagrammatic view of the device shown in Fig. l,exemplifying the manner in which the several electrodes are energizedwhen the device is utilized for certain of the purposes to which it isadapted,

Fig. 3 is a diagrammatic view of an alternative embodiment of ourinvention, and

Fig. 4 is a diagrammatic view exemplifying the utilization of ourelectric discharge device as a degenerative amplifier.

Referring now to Fig. 1 of the drawings, an electron discharge device,constructed according to our invention, may be constituted by an N-shape evacuated container 5 in one end of which is disposed an electronsource constituted by a photo-sensitive cathode 3 and in the other endof which is mounted an output electrode 5. It is, of course, to beunderstood that the cathode is shown mainly by Way of example, since theelectrons can be introduced into the device from a source exteriorthereof. A multiplying electrode 1 having a surface capable of secondaryemission is mounted within the container at the junction between thefirst leg 9 of the container and the connecting portion l 1 between itand the second leg i3, and a similar multiplying electrode i5 is mountedwithin the container at the junction between the connecting portion andthe second leg. Each multiplying electrode is so disposed that the axesof the leg and connecting portion adjacent thereto make substantiallyequal angles therewith.

For the purpose of concentrating accelerating and directing electronsfrom the cathode 3 toward the first multiplying electrode 7, anelectron-lens is disposed therebetween. This lens is constituted by aforaminous cylinder ll adjacent to the cathode and a conductive coating59 upon the inner wall of the container leg adjacent to the firstmultiplying electrode, to each of which appropriate potentials may beapplied.

As will be hereinafter shown more in detail, primary electrons emittedfrom the cathode are focused and directed toward the first multiplyingelectrode '1 by means of the electron lens just mentioned and therebycause the profuse emission of secondary electrons.

For the purpose of removing the secondary electrons and of giving theman initial velocity toward the second multiplying electrode 55, we mayinterpose, within the connecting member H of the tube, an acceleratingscreen element 2! to which a positive potential may be applied. In orderthat the secondary electrons from the first multiplying electrode may befurther accelerated and concentrated upon the second multiplyingelectrode 15, we interpose a second electron lens, constituted by aplurality of spaced apart conductive wall coatings 23 and 25, betweenthe first multiplying electrode and the second multiplying electrode. Tothese coatings the proper potentials may also be applied, as will laterbe shown.

The screen is not an absolutely essential of our improved device. If itis omitted, the first element of the electron lens adjacent to themultiplying electrode, if maintained at a higher potential, will serveto remove the secondary electrons and direct them toward the nextmultiplying electrode.

If desirable, the second leg 83 of the container may be replaced by asecond connecting tube provided with an additional accelerating screen,analogous to the screen element 2!, as well as suitableelectron-lens-forming elements, and the secondary electrons from thesecond emitter may again be concentrated and directed toward a thirdmultiplying electrode. For the purpose of simplifying the drawings,however, we have omitted further multiplying electrodes and have merelyshown the output electrode 5 as adjacent to the secondary emitter. Wewish it also to be clearly understood that the elements constituting theelectron-lenses may be disposed exteriorly of the container withoutdeparting, in any way, from the spirit of our invention.

In the device as actually constructed, the photo-sensitive cathode isconstituted by a disc of pure silver about three-quarters of an inch indiameter and ten-thousandths of an inch thick. The foraminous cylinderis made of nickel 32 mesh screen, which material was also utilized forthe accelerating screen electrode. The multiplying electrodes or targetsare also of pure silver, substantially one inch in diameter andtenthousandths of an inch thick. The output electrode is preferably madeof tantalum or an analogous metal three-quarters of an inch in diameterand five-thousandths of an inch thick.

. Referring now to Fig. 2 of the drawings, in the operation. of thedevice the photo-sensitive electrode, the several multiplyingelectrodes, the accelerating screen and the electron-lens elements maybe supplied with suitable potentials from any available source of directcurrent. This source is exemplified in the drawings by a potentialdivider 26 to the negative terminal of which is connected the cathode 3and to the positive terminal of which, through an output resistor 28, isconnected the output electrode 5. From an inspection of Fig. 2, it willbe noted that, starting with the cathode, each element is connected to asuccessively more positive point upon the potential divider. Such modeof connection we have found to be advantageous but it is not to beinferred from the drawings that it is obligatory, since it is entirelypossible to maintain each multiplying electrode at the lower po tentialof the lens element next adjacent thereto and accelerating screen mayalso be omitted. Alternatively, the cylinder I! may be maintained at thepotential of the cathode 3, if desired.

Since the foraminous cylinder I! is an element of the firstelectron-lens adjacent to the oathode, it, of course, could be replacedby a conductive coating on the inner wall of the tube. The form shown,however, is advantageous in that it permits the ingress of light fromany suitable source. This source might be modulated by a movingphotographic sound record or it might be any other source of lightcorresponding to the fluctuations of which an amplified electric currentis desired.

In order that our disclosure may be complete, we shall now give thesequence of operations in manufacturing the actual device, shown in Fig..lv

of the drawings.

The glass envelope was first fabricated in the form shown, the ends ofthe legs and the junctions between the legs being left open in order topermit the sealing therein of the respective electrode-supportingpresses.

After the glass blank was prepared, the positions of the metalliccoatings on the glass were marked off and, on the inside of the blank,Was deposited a commercially available platinizing solution followingthe marks previously made. The whole blank was then heated toapproximately 400 centigrade in order to reduce this platinizingsolution to metallic platinum. The electrodes, previously described,were mounted on the wires of the glass presses which. were then sealedin place. The tube was sealed to a high vacuum system by means of atubulation (not shown), through which the tube could be evacuated. Anappendage (not shown) containing pellets of a caesium compound, such ascaesium chromate, and a reducing agent, such as aluminum powder orsilicon powder, was sealed onto the tube by means of another tubulation(not shown) through which the caesium from the pellets could be admittedto the main body of the tube.

The tube was then baked at 450 centigrade, being evacuated at the sametime. The bake continued for approximately thirty minutes after the ovenreached the final temperature. After the baking, the tube was cooled anda small amount of pure oxygen was introduced into it at a pressure ofapproximately 1 mm. of mercury. The cathode and multiplying electrodeswere next oxidized by passing an electrical discharge from theseelements to some other element in the tube until the electrode surfacesacquired a bluish-green tinge. The oxygen was then pumped out and thepellets of caesium compound and reducer were heated sufficiently tostart the reaction which yields metallic caesium. The metallic caesiumwas driven, by means of heating the appendage, into the main body of thetube. The tube was once more baked at 200 centigrade for approximatelyten minutes and allowed to cool. The caesium appendage was then sealedoff the tube and the tube sealed off the vacuum system.

It might be thought that the caesium would be deposited upon thelens-elements and the electrodes other than those which are intended forprimary or secondary emission. Such is undoubtedly the case, but, byreason of the greater affinity of caesium for an oxide, particularlysilver oxide, when the tube was heated in the final heating, most of thecaesium was driven oii the other elements and was taken up by theoxidized silver electrodes. The caesium forms a chemical compound withthe silver oxide which is reasonably stable, although the actualchemical reaction that takes place is not definitely and accuratelyknown.

From the foregoing description of the device shown in Fig. 1 of thedrawings, it is, of course, possible to infer that the zig-zag shape isnecessary. Such is not the case, however, since the elements themselvesmay be arranged in a zigzag path within a cylindrical container. An alternative device of such type is exemplified by Fig. 3 of the drawings,wherein elements corresponding to those shown in Fig. 1 are similarlydesignated.

It should also be understood that We are not limited to the use of aphotosensitive cathode, since it lies within the scope of our inventionto replace it by any suitable electron source, such as one of thethermionic type, and to provide one or more grids for the purpose ofsuitably controlling the electron emission. The manner in which acontrollable thermionic source could be mounted in the container in lieuof the photosensitive cathode 3 will be perfectly apparent to thoseskilled in the art and no necessity is seen for illustrating it. Iffurther information, however, is desirable, attention is directed towardthe copending application, Serial No. 4,049, filed January 30, 1935, inthe name of Louis Malter, and assigned to Radio Corporation of America.

Referring once more to Fig. 2 of the drawings, when light, steady orfluctuating in character,

falls upon the photo-sensitive cathode 3, primary electrons are emittedtherefrom in random directions. By reason of the positive potentialapplied to the first multiplying electrode, the primary electrons areaccelerated toward it. In order to make certain that substantially alloi the primary electrons reach the first multiplying electrode, theforaminous cylinder I? and the first interior coating l8 are maintainedat successively higher positive potentials with respect to the cathode.In such case, the space between the cylinder and the coating function asconverging electron-lens to cause the random secondary electrons to befocused upon a definite portion of the first accelerating electrode. Thesecondary electrons emitted from said electrode are given an initialacceleration toward the second multiplying electrode by means of theinterposed screen 2!. They are directed and focused upon the secondmultiplying electrode E55 by means of the second electron-lensconstituted by the intermediate coatings 23 and 25, in the same manneras described in connection with the primary electrons. Upon reaching thesecond multiplying electrode the secondary electrons drive out furthersecondary electrons which are drawn over to the output electrode andgive rise to current in the output resistor.

Our improved device, in addition to its capability of providing afluctuating current in response to a fluctuating light source, is alsocapable of giving non-linear amplification. That is to say, in eitherthe photosensitive type or the thermionic type, output current may beobtained which is not directly proportional to input excitation butwhich follows some other curve. Non-linearity may be, for example,obtained by interposing an impedance device 2'5 between the potentialdivider and the first multiplying electrode T. This impedance device maybe a resistor or an inductor having such electrical characteristics thatwhen the current therethrough changes, the potential on the secondaryemitting electrode also changes to alter the gain in proportion thereto.An inductor, of course, would be preferable if a characteristicnon-linear with re spect to frequency is desired.

We have also discovered that our improved device may be so operated adegenerative amplifier that a variation in the output current therefromcauses a corresponding variation in the primary electron stream in suchdirection as to oppose the original stream-variation which causes thesaid output variation.

We have not thoroughly investigated the possibilities of our device inthe aforementioned direction but it could probably be used for thepurpose of regulating a power supply or for some analogous purpose.

The manner in which our device may he utilized as a degenerativeamplifier or as a non- Y fluctuating source of electrons is exemplifiedby Fig 4 of the drawings, wherein elements analogous to those shown inFigs. 1, 2 and 3 are similarly designated.

Referring to Fig. l, it will be noted that a resistor 29 is interposedbetween the potential divider 26 and the second multiplying electrode. Asimilar resistor 3i is interposed between the potential divider and thephotosensitive cathode, the two resistors being coupled through acapacitor 33. A signal to be amplified may be applied to the screenelement 2lA interposed between the second multiplying electrode l5 andthe output electrode 5 as, for example, by means of an input element,such as a resistor 35, included between the otential divider and thescreen. Assuming that the light which energizes the cathode 3 suffers anundesired momentary increase in intensity, this gives rise to (a)increased. photo-emission from the cathode, (b)

increased secondary emission from electrodes 1 and I5 and (c) anincrease in the flow of electrons to electrode l5 through resistor 29.The increase in the flow of electrons to electrode l5, through resistor29, causes, an increase in the already positive potential thereon and,since electrode [5 is coupled to the cathode 3 through capacitor 33,this change in voltage will be reflected in an increase in the potentialof the cathode. This increase in the potential upon the cathodedecreases the potential between the cathode 3 and the first multiplyingelectrode 1. As a result the primary electrons strike electrode 1 with alower velocity causing a decrease in the ratio of secondary electrons toprimary electrons, and hence a decrease in the number of electronsstriking electrode l5. This compensates for the previously describedincrease in the number of electrons striking electrode I5 due to theundesired momentary increase in the intensity of the light whichenergizes the cathode. The electron stream, therefore, between thesecond multiplying electrode l5 and the output electrode 5 is responsiveto the input signal only and its magnitude is thus renderedsubstantially independent of light fluctuations.

If amplification of the controlled secondary electron stream is desired,additional multiplying stages may be interposed between the secondmultiplying electrode and the output electrode in the manner indicatedby Fig. 3.

Obviously, an analogous system could be employed in connection with adevice of the thermionic type but since the connections are obvious, nonecessity is seen for further illustration.

Our improved device, either of the photosensitive cathode or thethermionic cathode type, is capable of generating sustainedoscillations. This be accomplished through utilization of any of theconventional regeneration circuits which permit of feeding back aportion of the output potential to the input circuit in proper phase.

ight should not be lost, also, of the fact that our device is capable ofuse as a demodulator. Such purpose may be accomplished by applying thesignal to be demodulated either across the impedance device 21, shown inFig. 2, or in series therewith.

Many other uses of our device'will be apparent to those skilled in theart, such, for example, its use as a combined oscillator-modulatorthrough the application of modulating potentials to one of the targetswithin the tube while the carrier frequency is applied to the inputcircuit or while the device is in the self-oscillatory condition. Sinceit is substantially impossible to list herein all of the possibilitiesof our improved device, it is to be distinctly understood that ourinvention is not to be circumscribed by the examples given.

By reason of the shape of our improved electric discharge device and thepresence of the electron lenses, electrons from any given source areprevented from being driven past their preallocated target and arecaused to impinge thereon.

Because of the fact that the acceleration of the electrons is producedthrough the action of an electric field which does not interfere withthe focusing action of the electron-lenses, the space charge limitationsof prior devices are avoided. The efficiency thus obtained is muchgreater than where dependence is placed solely upon electrostatic fieldsbetween adjacent electrodes for the purpose of both accelerating theelectrons and directing them to the secondary emitters. By providing theelectron lenses the impinging electron stream is concentrated anddirected to the emitting surface and at the same time there may bemaintained at the surface an electrostatic potential gradient which isfavorable for removing the emitted electrons with maximum efiiciency.Substantially no interference or interruption between the high velocityimpinging electrons and the low velocity secondary electrons has beenobserved and it seems highly probable that there is no actualinterference between the concentrating fields and the acceleratingfields.

It follows from the foregoing enumerated facts that in our improveddevice the output current is not limited by space charges. As a result,the device appears to have no saturation point, the amount of outputcurrent which it can provide being only dependent upon the amount ofheat which the electrodes can dissipate, upon their resistance todestructive electrostatic forces, the potentials applied to theelectrodes and the adjustment of the focusing action of the electronlenses through suitable control of the potentials applied thereto.

We are aware of many physical modifications of our device and many otherpossible uses thereof that at once will be apparent to those skilled inthe art. Our invention, therefore, is not to be limited except insofaras is necessitated by the prior art and by the spirit of the appendedclaims.

We claim as our invention:

1. An electric discharge device comprising an evacuated envelope, anelectron-source, a secondary emitter-electrode, an acceleratingelectrode and a target electrode mounted in the order named within saidenvelope, said emitter-electrode being accessible to electrons from saidsource and said accelerating and target electrodes being accessible tosecondary electrons from said emitter-electrode, and a plurality ofapertured electrodes serially disposed between said emitter and targetelectrodes for focusing said secondary electrons upon said targetelectrode, the apertures being substantially symmetrical with respect toa line drawn between said emitter, accelerating and target electrodes.

2. An electric discharge device comprising an evacuated envelope, anelectron source, a secondary emitter electrode and a collector-electrodemounted in spaced relation in the order named within said envelope, saidemitter-electrode being accessible to electrons from said source andsaid collector-electrode being accessible to secondary electrons fromsaid emitter electrode, and means comprising an accelerating electrodeand a plurality of electrically separate electron-lens elements mountedin the space between said emitter and collector-electrodes for directingthe electrons in their passage therebetween.

3. The invention as set forth in claim 2 and wherein said electricallyseparate electron lens elements are of the electrostatic type andwherein said accelerating electrode is mounted intermediate two of saidelectrostatic electron lens elements.

4. An electric discharge device comprising an evacuated envelope, aplurality of electrodes capable of secondary emission mounted in spacedrelation within the envelope and means comprising an acceleratingelectrode and a plurality of electron lens elements mounted in the spacebetween two adjacent secondary emitter electrodes for guiding secondaryelectrons from one electrode to the other electrode of the pair.

5. An electric discharge device comprising an evacuated envelope, aplurality of electrodes capable of secondary emission and an outputelectrode mounted in spaced relation in said envelope, an acceleratingelectrode and a plurality of electron lens elements mounted in the spacebetween two adjacent secondary emitter electrodes for directingelectrons therebetween, and a second accelerating electrode mounted inthe space adjacent said collector electrode for drawing the electronsfrom the preceding secondary emitter electrode thereto.

GEORGE A. MORTON. LESLIE E FLORY.

